In the well known “lost wax” process of investment casting, a fugitive or disposable wax pattern is made by injection molding melted wax or other fluid fugitive material in a die corresponding to the configuration of the article to be cast. A plurality of such molded patterns are joined to a common gating system made of wax or other fugitive material, and/or to one another by gating, to provide a pattern assembly that can be invested in a ceramic shell mold. For example, the gating system typically includes a sprue or runner to which one or more patterns are connected and a pour cup connected to the sprue or runner. Multiple fugitive patterns may be joined by gating in end-to-end or other arrangement to one another before being joined to the gating system. The actual configuration of the pattern assembly varies depending on the type of cast article to be made in the shell mold; for example, whether the cast article is to be an equiaxed grain, directionally solidified columnar grain, or single crystal casting.
Typically, as a result of complexity of the pattern assembly and/or cost of injection dies, the patterns are joined to the gating system and/or to one another manually using labor-intensive techniques in a manner that requires a joint that is waterproof and structurally sound to prevent the patterns from separating from the gating system as the shell mold is built up on the pattern assembly. When the patterns and gating system are made of wax, the patterns are joined to the gating system by wax welding processes well known in the art to produce a wax weld joint between the patterns and the gating system. However, manually assembled fugitive pattern assemblies can exhibit unwanted variability in pattern locations and joint quality from one pattern to the next and from one pattern assembly to the next.
The pattern assembly thus produced is invested in a ceramic shell mold by repeatedly dipping the pattern in a ceramic slurry, draining excess slurry, stuccoing with coarse ceramic particles or stucco, and air drying until a desired thickness of a ceramic shell mold is built-up on the pattern assembly. The pattern assembly then is removed from the green shell mold typically by heating the shell mold to melt out the pattern assembly, leaving a ceramic shell mold which then is fired at elevated temperature to develop appropriate mold strength for casting a molten metal or alloy.
An object of the invention is to provide an improved method of making a fugitive pattern assembly, as well as the pattern assembly made by the method, useful in making shell molds for the lost wax precision investment casting process wherein the need for manual joining of the fugitive patterns to one another and/or to a gating system is reduced.